Many systematic studies of dark clouds and low-mass protostellar cores have shown the HNC/HCN ratio to be close to unity [1]}, [2]}, [3]}. No difference is observed between the values measured in prestellar and protostellar cores, implying that the evaporation of HCN and HNC from dust grains does not contribute significantly to the observed emission in the cold envelope. On the contrary, towards the high-mass star forming region OMC-1 in Orion, the HNC/HCN ratio displays strong variations with especially low values \(\approx 0.01\)  towards the hot core regions while it is of the order of 0.2 in adjacent ridge positions. While the abundance of HCN is similar to that of dark cloud cores, the HNC abundance is 2 orders of magnitude lower in the high-temperature gas of the hot core [4]}. A somewhat similar behaviour is observed towards IRAS16293\(-\) 2422 when looking at the high-excitation lines of HCN and HNC [5]}.
Recently, [6]} demonstrated the high sensitivity of the HCN/HNC \(J\) =1–0 line intensity ratio to the gas kinetic temperature.
